Deforestation is the conversion, long term, of areas of forested land to non-forested land through human activity. (Where cleared areas are quickly replanted, it is not regarded as deforestation.) Reforestation is the reversal of that process - i.e. planting trees on land that has historically been forested but has recently been used for other purposes. A less commonly seen term, afforestation, refers to the process of establishing forests on land that has never been forested, or not in recent history.